The customers who receive a ‘Make Someone Smile’ sleeve with their drink are encouraged to pass this to a friend, colleague or a stranger who can redeem it for a free coffee or tea before the end of February.

The ‘Make Someone Smile’ promotion ties into the firm’s Random Acts of Kindness policy, which was widely publicised in April last year and is Pret's alternative to traditional loyalty card system. This gives each shop a designated amount to spend on rewarding its customers, with staff free to choose who they give out free food and drinks to, be it regular customers they like – or someone they fancy.

The chain – which has 400 shops, the majority in the UK – had over 4,000 messages of support on social media during 2015 relating to its scheme.

Speaking about today's launch, chief executive Clive Schlee said: "We often get feedback from customers about how our shop teams have brightened up their day. Our teams love being able to make customers smile, and we hope the recipients of our special coffee sleeves enjoy being able to pass that joy on to somebody else themselves."

For its Christmas campaign, Pret wrapped up its shop in London's Broadwick Street as a giant present, and invited customers to tear their way through to get a free festive sandwich. The chain then 'donated' its marketing budget to charity, raising awareness and money for five homelessness charities.